The invention relates to a blind rivet. A blind rivet is one that can be inserted and set by access to one side only of aperture members to be riveted together.
More particularly, the invention relates to a self plugging blind rivet comprising a tubular shell having a preformed radial head at one end, and a stem extending through the tubular shell and having a stem head adjacent the tail end of the shell. In use, the rivet is inserted, shell tail end first, through the apertures in the members to be riveted until the shell head contacts the nearer member and the shell tail protrudes beyond the further member. In order to set the rivet, the shell head is supported by an abutment and the stem is then pulled so that the stem head compresses the shell against the abutment and causes axial contraction of the shell and radial expression of at least part thereof to form a blind head which, together with preformed head, clamps the members together. At least part of the stem adjacent to its head is retained within the shell to plug the latter after the rivet has been set. The remainder of the stem (i.e. the tail end remoted from the head thereof) is preferably broken off to avoid protrusion from the shell head. This is generally effected by providing a breakneck at an appropriate position in the shank of the stem.
A rivet of this type is described in WO 98/23872 and features that are important in the performance of such a rivet are described in that application. The application also describes how a rivet can be constructed in which the portion of the river stem retained in the rivet after placing fully plugs the bore of the river and breaks flush with the head of the rivet under all working conditions of sheet thickness. To this end the pintail has reduced diameter relative to the stem shank and the breakneck is formed at the junction of the stem shank and the pintail.
In a first aspect the present invention provides a self plugging blind rivet for securing together apertured members, the rivet comprising a tubular shell having a tail end face at one end and a preformed radially enlarged head at the other end, and a stem that extends through the shell and has a stem head adjacent the tail end face of the shell and a breakneck located away from the stem head, in which the region of the stem shank on the stem head side of the breakneck has a plurality of depressions formed therein extending to the breakneck, the corresponding region of the shell substantially filling the depressions in a region adjacent the stem head and progressively less towards the breakneck whereby, in use and under axial compression loading of the shell, the shell preferentially buckles in the region adjacent the member remote from the preformed head to form a blind head in contact with said member so as to clamp together the apertured members.
The depressions are preferable axial grooves but may also be spiral or even annular. Especially where the depressions are axial or spiral, the stem shank and the shell may be provided with mutually-cooperating means to lock them together. Such means may comprise an annular groove formed around the stem shank, preferably immediately below the stem head, the groove containing shell material to retain the stem within the shell so that, before placing, the assembly of stem and shell is locked together and the part of the stem retained in the shell after placing is also locked to the shell. The groove may have an axial length of approximately one half of its diameter.
Preferably, the region of the stem shank having formed therein the depressions generally has a major diameter which is greater than the diameter of the region remote from the breakneck, whereby the end adjacent the breakneck comprises one or more radially-protruding shoulders which in use with a placing tool engage the nose of the placing tool so that the retained part of the stem breaks substantially flush with the rivet head throughout the working range of thickness of the members being riveted.
Preferably, the cross sectional area and strength of the stem in the region adjacent the stem head is substantially equal to that of the cylindrical part of the stem. The number of axial depressions may vary from a minimum of four depending on the method of manufacture. Preferably the distance across the mouth of the depressions is less than the circumferential width of the land areas between.
Preferably, the depressions are produced by a forming process so that material displaced from the depressions is contained in the lands between the depressions resulting in the increased diameter.
According to a further aspect of the invention a method is provided of assembling a rivet comprising a tubular rivet shell having a preformed radially enlarged heat at one end and a stem comprising a head and a shank which carries a breakneck remoted from the stem head, the region between the stem head and breakneck being formed with a plurality of depressions which extend to the breakneck, the method comprising inserting the shell in a die, a first part of which closely fits the outside diameter of the shell, a second part is formed as a taper and which progressively increases in diameter until it intersects a third part of the die into which the tail end of the shell protrudes, supporting the head of the shell with an abutment, inserting a punch which has a first diameter closely fitting the bore of the rivet shell and a second diameter which closely fits the third part of the die, applying a compressive force between the punch and the abutment so as to compress the rivet shell to fill both the taper and the larger diameter of the die, removing the abutment and punch, inserting a stem and ejecting the assembled shell and stem through the die whereby, during ejection, the material of the shell is forced into the depression in the stem to substantially fill the depressions at the tail end of the shell and progressively less towards the breakneck.
Preferably, the external diameter of the shell in the assembled rivet is equal to the diameter of the stem head. To this end the first die bore should have the same diameter as the stem head, although this may be difficult to arrange in practice. Consequently it is preferable that the stem head before assembly of the rivet is slightly larger than the first die bore so that, when the assembly is ejected, the stem head is sized to the diameter of the first die bore and hence to the diameter of the shell.
As an alternative to using a separate punch to form the rivet shell to the shape of the die, it is possible to approximate to this by using the stem in place of the punch to compress the shell against the abutment, thereby substantially filling the die, removing the abutment and ejecting the assembled stem and shell through the die.
Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: